1. Field of the Invention
This invention relates to the recovery of mineral values from subterranean formations and more particularly to a process for leaching subterranean mineral deposits to recover the mineral values.
2. Description of the Prior Art
The in situ leaching of mineral values from subterranean deposits is well-known in the art as a practical and economical means for recovering certain elements such as uranium, copper, nickel, molybdenum, rhenium, vanadium and the like. Basically, solution mining is carried out by injecting into the subterranean deposit a leaching solution which will solubilize the mineral values desired to be recovered, and the solution and solubilized mineral values, i.e., the pregnant liquor, are then recovered from the deposit for subsequent separation of the mineral values.
The cost of the chemicals employed in solution mining processes is a major factor in determining the economic viability of the process. Besides the leaching solution itself, oxidants, such as hydrogen peroxide or oxygen-containing gases, are often injected to enhance the leaching of those minerals which are only soluble, or are more readily soluble, in their oxidized forms. The solution mining processes of the prior art typically involve the injection of the oxidant continuously throughout the process. On the other hand, U.S. Pat. Nos. 3,278,233 and 3,708,206 disclose the injection of an oxygen-bearing gas to displace ground water from the formation prior to injection of the leaching solution. The large quantity of oxidant required in some of these processes can be economically prohibitive.
Depending upon the nature of the subterranean deposit, the typical leaching olution may be an acid, for example, an aqueous sulfuric acid solution or may comprise an alkaline carbonate solution. In view of the high proportion of carbonates typically present in many subterranean formations, the use of the known acid solutions is usually economically prohibitive because of the excessive consumption of acid due to carbonate solubilization. Consequently, alkaline carbonate leaching solutions are preferred over acid solutions for solution mining operations when carbonates are present in the formation.
Various solution mining processes involving the use of alkaline carbonate leaching solutions are disclosed in the prior art, for example in U.S. Pat. No. 3,708,206, using an aqueous ammonium carbonate solution and an oxidizing agent and U.S. Pat. No. 2,896,930, utilizing an aqueous solution of an alkali metal carbonate and hydrogen peroxide. Typically, the alkaline carbonate leaching solutions are maintained at a pH of about 8.2 to about 8.3. However, alkaline carbonate leaching solutions can present problems with respect to reduction of injectivity of the leaching solution and consequently reduced mineral production. Reduced injectivity can generally be ascribed to two major factors. The first factor is the swelling of formation clays by the alkaline solution. A second major factor is the exchange of cations, such as sodium, from the leaching solution with the calcium ions of the formation which results in the production of calcium carbonate in the leaching solution. Calcium carbonate, at the alkaline pH's utilized, precipitates from the leaching solution thus reducing the permeability of the formation, well injectivity, and causes scaling problems in the pumps and other ancillary equipment utilized in the process.
Another problem which arises from the use of the prior art alkaline carbonate leaching solutions, particularly the ammonium carbonate and/or ammonium bicarbonate leaching solutions, involves the restoration of the ground water in the subterranean formation to its preleach condition once the solution mining process has been completed. The cations introduced into the formation by these leaching solutions are ionically exchanged into the formation matrix and must later be removed. The removal of these exchanged cations is often difficult and, in the case of the ammonium ion, has been found to require a prohibitively long restoration period.
The present invention overcomes the foregoing deficiencies by providing a process for the recovery of mineral values from subterranean formations in which the loss of permeability and production associated with the alkaline carbonate leaching solutions is avoided. Also, the method of this invention requires significantly less oxidant than the prior art processes and therefore is more economically feasible.